spectral uv measurements in austria from 1994 to 2006

Atmos. Chem. Phys., 8, 7033–7043, 2008www.atmos-chem-phys.net/8/7033/2008/© Author(s) 2008. This work is distributed underthe Creative Commons Attribution 3.0 License.

AtmosphericChemistry

and Physics

Spectral UV measurements in Austria from 1994 to 2006:investigations of short- and long-term changes

S. Simic, P. Weihs, A. Vacek, H. Kromp-Kolb, and M. Fitzka

Institute of Meteorology, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria

Received: 3 January 2008 – Published in Atmos. Chem. Phys. Discuss.: 8 February 2008Revised: 2 September 2008 – Accepted: 14 October 2008 – Published: 5 December 2008

Abstract. The influence of variability of atmosphericparameters on short- and long-term changes of spec-tral UV irradiance measured at the Sonnblick observatory(47.03◦ N, 12.57◦ E, 3106 m) during the period from 1994to 2006 is studied. Measurements were performed withthe Brewer #093 single-monochromator spectrophotometerand with a Bentham DM 150 spectroradiometer (double-monochromator).

The influence of ozone, albedo, snowline and clouds onUV variability is evaluated for each parameter separately us-ing 10-year climatology. It is found that the effect of totalozone on short-term variability of UV irradiance at 305 nmcan be more than 200% and on average more than 50%.Clouds can cause variability of 150% or more and on aver-age 35%. Variability caused by albedo reaches a maximumof 32% in April (6% on average). In summer and autumn,total ozone and clouds strongly influence the variability ofUV radiation, whereas in winter and spring ozone has themore pronounced effect. A decrease in snowline height from3000 m to 800 m a.s.l. enhances the UV irradiance by a fac-tor of 1.24 for clear sky conditions and by a factor of 1.7 for8/8 cloud cover.

Long-term trends are investigated for the time period from1994 to 2006 based on clear-sky measurements, using thenon-parametric Mann-Kendall trend test. Significant down-ward trends (99% confidence level) are found for solar zenithangle 55◦ at wavelengths from 305 nm to 324 nm and ery-themally weighted irradiance according to CIE, which arecaused by an increase in sunshine duration during periods ofhigh total column ozone. Significant trends (90% confidencelevel) were also found for other combinations of wavelengthand SZA.

Correspondence to:S. Simic([email protected])

1 Introduction

UV irradiance reaching the earth’s surface is influenced bythe concentration of stratospheric ozone as well as by fur-ther atmospheric parameters, such as clouds, aerosols, andsurface albedo. Knowledge of spectral UV irradiance and itsdependence on these parameters, which may change in thefuture, are prerequisites to quantitatively understand and es-timate future UV radiation.

Increase of UV radiation during the last decades is re-ported where a decrease of stratospheric ozone has been ob-served (Kerr and McElroy, 1993; Zerefos et al., 1997, 2001;Bartlett and Web, 2000). The magnitude of this change andtheir causes are, however, uncertain, calling for more detailedinvestigations of the influence of clouds, albedo, and other at-mospheric parameters on UV radiation (Kerr and Seckmeyeret al., 2003; Bais and Lubin et al., 2007).

Several publications focus on the detection of long-termchanges in UV doses (Herman et al., 1996; Seckmeyer et al.,1997; Weatherhead et al., 1997; Lindfors et al., 2003). Thefirst spectrally resolved routine measurements started in the1990s, thus studies on long-term changes in spectral UV irra-diance are hampered by the limited number of years of avail-able data (Zerefos et al., 1997; Lakkala et al., 2003; Glandorfet al., 2005).

Clouds can cause strong variability of surface UV radi-ation (the factor by which incoming surface UV irradianceis enhanced or diminished due to changes in one of the in-fluencing parameters) and limit the detectability of ozone-induced trends in UV radiation (den Outer et al., 2005; Glan-dorf et al., 2005; Seckmeyer et al., 2008). Change in cloudcover caused by the global climate change is especially im-portant for the estimation of future UV radiation. Reuder etal. (2001) show that reduced cloud cover during summer canincrease UV radiation up to 15%.

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7034 S. Simic et al.: Spectral UV in Austria

Albedo is of great importance, especially in snow-coveredmountainous regions. UV irradiance is strongly enhancedwith snow covered ground due to multiple reflections be-tween ground and atmosphere, even more under overcast-sky conditions because of increased atmospheric backscat-tering through clouds. McKenzie et al. (1998) showed that,due to snow, UV-B increased by about 30% under clear skyand about 70 % under cloudy sky conditions at Lauder, NewZealand. Kylling and Mayer (2001a) investigated the in-fluence of snowline on UV irradiance at 340 nm, studyingsnow-free conditions and then moving the snowline from1000 to 0 m. With decreasing snowline, enhancement in UVof 23–27% for cloudless sky was obtained, while for overcastconditions the enhancement was around 40–60%.

Different methods are available in literature that separatethe different influencing factors causing short- and long-termchanges in UV radiation (Fioletov et al., 2001; Kylling etal., 2001b; Arola et al., 2003). Earlier investigations showthe effect of ozone on UV radiation, whereas analyzing theinfluence of clouds and albedo is more difficult. Clouds showstrong variability, and measurements of albedo which may beused as model input data are hardly available.

In this study, the influence of clouds, ozone, and surfacealbedo on spectral UV radiation at the Sonnblick observatory(3106 m) is investigated using continuous spectral UV irra-diance measurements and model calculations. The influenceof the different factors on short-term UV variability is shownusing the available UV data sets. The method to separatethe effects of clouds, ozone, and surface albedo is adaptedfrom Arola et al. (2003) who used this method to analysemeasurements at two stations, at Sodankyla, Finland (67◦ N)and at Thessaloniki, Greece (40◦ N). Spectral UV irradiancetime series are analyzed for possible trends. Measurements atSonnblick observatory starting in 1994 represent the longesttime series of spectral UV irradiance data in Austria.

2 Method

Monitoring of spectral UV radiation and total ozone atSonnblick observatory in Austria is performed since 1993.The Sonnblick observatory (47.05◦ N, 12.95◦ E) is in south-west Austria at the border between Salzburg and Carinthiaon a mountain top at 3106 m altitude. It is surrounded byrock faces on the northern side and by a glacier on the south-eastern side. The area shows a very pronounced topography.The valley adjacent to Sonnblick is 1300 m lower than thesummit. Nearby summits in this region are at approximatelythe same altitude. 16% of the surrounding area is coveredwith glaciers and in winter 88% of the surface is coveredwith snow under the assumption that no snow covers the rockfaces (Weihs et al., 1999).

The Sonnblick observatory is a station of the AustrianWeather Office that monitors synoptic and climatologicalmeteorological data. They also record the cloud and ground

surface conditions. The cloud fraction above and below theobservatory as well as the cloud types are visually observedsix times a day. Snowfall and the depth of the snow cover aremeasured once a day. Snowline is monitored by the weatherobservers. All this data were used in this study.

2.1 Instrumentation and measurements

Spectral UV measurements at Sonnblick observatory havebeen performed with a Brewer spectrophotometer (singlemonochromator) since 1993 and a Bentham DM 150 spectro-radiometer (double monochromator) since 1997. The Brewerspectrophotometer is used to measure global UV irradiancein the spectral range from 290–325 nm using a step widthof 0.5 nm and to measure total column ozone. Two inde-pendent calibrations are performed on a regular basis (1–2 months) with a portable 50 W lamp obtained from SCI-TEC company and with a self-built portable 1000 W lamp-assembly which is calibrated to another NIST (National In-stitute of Standards and Technology) calibrated 1000 W lampin our laboratory. The Bentham spectroradiometer is usedto measure global UV irradiance in the spectral range 280–500 nm with a step width of 0.5 nm. It is calibrated regularlyto the self-built 1000 W lamp-assembly, a NIST calibrated1000 W lamp and a PTB (Physikalisch-Technische Bunde-sanstalt, Germany) calibrated 1000 W lamp. Comparisons ofthe Brewer #093 with the travelling standard (Brewer #017)and with a Bentham DM 150 spectrometer are performedregularly. Comparisons of total ozone measurements withthe standard Brewer-Spectrophotometer #017 are availablesince 1993 and show a deviation of less than 1%, and mea-surements in the UV-B range are within±5%. Measure-ments in the UV-B range vary by±7% at 305 nm and by±4% at 320 nm compared to the Bentham spectroradiome-ter.

The investigations of short and long-term changes wereperformed with data of the Brewer only. Additional analysislike the influence of snowline on UV-irradiance, is based onBentham data.

2.2 Short-term changes in UV-B irradiances

The analysis is based on UV irradiance measurements per-formed with the Brewer spectrophotometer. Radiative trans-fer model calculations were used for the estimation of effectsof the contributing factors on the observed variability of UVirradiance. Simulations were carried out with the radiativetransfer model SDISORT developed by Stamnes et al. (1988)to determine the influences of ozone, surface albedo, aerosolsand clouds on UV variability. In a first step, model calcula-tions based on the actual input data solar zenith angle, to-tal column ozone, surface albedo, clouds and aerosols wereperformed to reconstruct the actually measured data of theBrewer.

Atmos. Chem. Phys., 8, 7033–7043, 2008 www.atmos-chem-phys.net/8/7033/2008/

S. Simic et al.: Spectral UV in Austria 7035

Fig. 1. Measured enhancements in UV irradiance at 305 nm due to changes in snowline under clear sky (left) and overcast conditions(right). Enhancement due to snowline at a specific altitude is the ratio of UV irradiance with snowline at that given altitude compared to UVirradiance with snowline at 3000 m a.s.l.

These comparative calculations were performed for mea-surements at a solar zenith angle of 63◦ for the entire pe-riod from 1994 to 2003. Then model calculations usingclimatological and actual input data of the same period wereperformed at the same SZA.

The daily climatology of each input variable for each dayis estimated on the basis of the 10-year data. To calculatethe 10-year climatological value of ParameterX on Julianday Y (JD Y ), the values ofX are summed up through theyears 1994–2003 and then are divided by the number of years(Arola et al., 2003). The formula is given below:

X (JDY ) =

2003∑i=1994

X (JDY )i

10(1)

The influence of a specific parameterX on UV radiation (i.e.the variability of UV irradiance due to parameterX) is thenestimated as the ratio of the modelled radiation using the ac-tual value of parameterX and climatological data of the re-maining variables to the modelled radiation using climaticdata of all variables, including parameterX. The amplitudeof each parameter is calculated using Eq. (2):

A =max. ratio out of all years−min. ratio out of all years

mean ratio over all years(2)

With this definition, the amplitudeA can be regarded asthe maximum possible variability in surface UV irradiancecaused by parameterX. This determining quantity is com-puted for each influencing parameter. The averaging periodwas set to one day for short-term and to one month for long-term variability. Additionally, the standard deviation is cal-culated and regarded the representative quantity for the meanvariability during the averaging period.

2.2.1 Model input data

Ozone

Daily means of total column ozone from Brewer measure-ments are the input data to reconstruct the measured UVdata. The daily climatology of ozone is calculated as theaverage of the ozone measurements from 1994 to 2003 ac-cording to Eq. (1). Diurnal variations of total column ozonecan be as high as 30% of the daily mean during late winterand spring and up to 10% during summer and fall (Simic,2006). Since these fluctuations do not represent climatologi-cal changes and make comparison to modelled spectra basedon daily data almost impossible, it was decided to furtherreduce the high variability in the daily climatological meanvalues of total column ozone by applying a moving-averagefilter with 11 day period (Julian dayY±5). This was donefor total column ozone only and not for any other input data.

Albedo

An algorithm is introduced that uses following routine ob-servations of snow condition: Snow height, time since lastsnowfall and snow line (the latter being defined as the loweraltitudinal boundary of the snow-covered area, its altitude isgiven in meters above sea level). These data were obtainedfrom the Austrian Weather Service and were used to estimatethe effective surface albedo in the UV range on a daily basis.

Figure 1 shows the average enhancements of irradiance at305 nm with decreasing snowline for clear-sky and overcastconditions. The ordinate shows the ratio of the irradianceat different snowlines and the mean irradiance at a snow-line of 3000 m. For clear sky conditions this ratio is 1.24(±0.04) at snowline of 800 m and 1.14 (±0.03) at snowline1500 m. Cloudiness enhances the influence of the albedodue to multiple reflections between the surface and the lowerbond of the clouds. This effect was also studied by Wuttke

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Fig. 2. Calculated daily climatological mean of effective albedo atSonnblick observatory, computed with Eq. (1), 10-year period.

and Seckmeyer (2006) for conditions in Antarctica. Theyshowed that UV zenith sky radiance may increase by morethan a factor of 2, relative to clear sky conditions, due to thecombined effect of clouds and high snow albedo. 600 mea-surements with 8/8 cloud cover were included in Fig. 1 torepresent overcast conditions. Under 8/8 cloud cover, UV ir-radiance at 305 nm is enhanced by a factor of 1.7 when thesnowline is 800 m instead of 3000 m.

Following the method of Schwander et al. (1999) the “ef-fective albedo” that affects the surface UV irradiance, is de-rived by parameterisation based upon snow height and timesince last snowfall, and by multiple linear regressions be-tween snow condition and albedo, resulting from a best fit ofmodelled and measured UV irradiances at Sonnblick obser-vatory. This method improves the accuracy of the UV irradi-ance calculations significantly, compared to a method whichuses an albedo averaged over the area of interest. Regressionanalysis delivered parameterisation of the “effective” albedoA according to Eq. (3):

A=0.659−2.04·10−4·G+4.97·10−3

·N−3.23·10−3·T (3)

In Eq. (3),G is the snow line (in m a.s.l.),N is the depth ofthe fresh fallen snow (in cm) andT is the time since the lastsnowfall (in days). Fresh fallen snow increases the effectivealbedo whereas the albedo decreases with increasing snow-line and days after the snowfall. 500 cases were included forcalculating regression Eq. (3). The explained variance of thisparameterisation is 70%, and the effective albedo varies from0.02 to 0.89. Figure 2 shows calculated values of the effec-tive albedo at Sonnblick observatory. Climatological meanvalues of the albedo determined for the 10 year period are0.3–0.6 during winter and spring, and 0.09–0.25 during sum-mer. Effective albedo of 0.63–0.78 determined for snowline800 m is comparable to the model calculations presented by

Fig. 3. Monthly means of measurements of aerosol optical depth at306.3 nm at Sonnblick observatory.

Weihs et al. (2000). Rengarajan et al. (2000) measured thealbedo at the Sonnblick observatory in winter. Their exper-imentally determined values in the range from 0.73 to 0.78are well comparable to the albedo values for UVA and visi-ble wavelengths determined in this study at a low snow lineof 800 m a.s.l.

Aerosols

Observations of direct solar radiation with the Brewerspectrophotometer at five channels 306.3 nm, 310.1 nm,313.5 nm, 316.8 nm and 320.1 nm were used for the aerosoloptical depth (AOD) calculation. Figure 3 shows monthlymeans of the aerosol optical depth at 306.3 nm in the pe-riod from 1997 to 2002. Optical depths of aerosols at theSonnblick are 0.03–0.08. Using this range, the calculatedvariation of UV irradiation is about 3% at 305 nm (Weihs etal., 1999). These results underline the fact that aerosols havea small influence on UV irradiance in high mountainous re-gions.

Clouds

Model calculations in presence of clouds use the actually ob-served cloud cover. Cloud transmission data measured withthe Bentham DM 150 spectrometer and cloud observationsare used in the analysis. Ratios of measured UV intensitiesand modelled clear sky values (i.e. the cloud modificationfactor) for the actual zenith angle and ozone amount are usedto analyze the dependence of UV irradiance at several wave-lengths (305 nm, 315 nm, 370 nm) on cloud amount. Thestudy delivers dependencies of the cloud modification factoron total cloud amount, cloud type, wavelength, solar zenithangle and ground albedo, which are used in the present in-vestigation (Simic et al., 2006).



2.2.2 Comparison of measured and calculated absolute UVlevels

Calculations of the UV irradiation are performed to testwhether they can reproduce the actual irradiance measure-ments under clear sky condition. In Fig. 4 the ratios ofmeasured and modelled UV irradiances at 305 nm and solarzenith angle 63◦ are shown for the time period 1994–2003. Inthe period from 1994 to 2003, 95% of the calculated UV ir-radiances deviate less than±8% from the actually measuredones.

2.3 Long-term changes

Long-term changes are calculated for different wavelengths(305 nm, 310 nm, 315 nm, 324 nm and erythemally weightedirradiances according to CIE) and solar zenith angles (45◦,55◦ and 65◦) from spectra acquired with the Brewer spec-trophotometer. Time series from the Brewer have been cho-sen due to their extent back to 1994 and because of the com-pleteness with no gap longer than three or four weeks appear-ing throughout the whole period.

The average of the SZAs of the selected measurementsdoes not significantly change during the investigated period.It is therefore unlikely to introduce an artificial trend causedby increasing or decreasing SZAs within the data selection.Monthly mean values are calculated for each combination ofwavelength and solar zenith angle for the period from Jan-uary 1994 to December 2006 except for months with toofew data (less than five days per month) or too high cloudcover (N>3/8). As solar zenith angles as high as 45◦ are notpresent from November to February, these months had to beexcluded in the analysis of SZA 45◦. Measurements at solarzenith angles of 55◦ and 65◦ are available for all months.

The monthly climatological mean values where obtainedby averaging every month over the 13-year period of investi-gation. The relative departures from the long-term mean val-ues where computed by comparing monthly mean values tothe climatological mean values of the respective month. Thetime series were checked for the existence of trends usingthe non-parametric Mann-Kendall (MK) trend-test (Mann,1945; Kendall, 1975). The results of the MK test were testedagainst the hypothesis of no detectable trend, i.e. the databeing normally distributed. This was done at several sig-nificance levels fromα=0.1 down toα=0.001, whereα isthe probability of rejecting the alternative hypothesis erro-neously. The magnitudes of the underlying trends were es-timated using two methods of linear regression: the least-square method and the non-parametric Theil-Sen slope esti-mate (Hollander and Wolfe, 1999), the latter being less sen-sitive to outliers and missing values, which may occur in pe-riods of instrument maintenance and calibration and harshenvironmental conditions. The results are shown in percentper decade.

Fig. 4. Ratios of measured to modelled irradiances at 305 nm andsolar zenith angle 63◦ in the period from 1994 to 2003 presented ina box plot diagram. The boxes are bounded by the 25% and 75%quartile, whiskers denote the 90% percentile. Calculations are forcloudless conditions only.

To complement the spectra and to enable data filteringfor various atmospheric parameters and conditions, detailed3-hourly cloud and snow observations from the AustrianWeather Service at Sonnblick observatory were included andappended to the appropriate UV-measurements. All spectrawere checked and corrected with the SHICrivm algorithm bySlaper (2002). Additionally, the spectra were CIE-weightedand integrated.

It was decided to check for trends in clear-sky spec-tra (N≤3/8) only, thus limiting influencing factors on UV-irradiance to any other than cloud cover. Situations with totalcloud cover greater than three eighths were omitted. Furtherreducing the maximum allowed cloud cover (<3/8) wouldhave considerably lessened the number of available data-sets.To assure that no artificial trend was introduced by the selec-tion criteria, the partitioning of 0/8, 1/8, 2/8 and 3/8 cloudcover was checked for underlying trends over the period 1994to 2006. No significant change was found using the MK-test.Due to the decreased variations in time series of clear-sky ir-radiances as opposed to all-sky conditions, significant trendsare supposed to be more easily detected in relatively shorttime series, as proposed by Glandorf et al. (2005).

3 Results and discussion

3.1 Short-term changes

The influences of total column ozone and albedo on 305 nmirradiance during the period 1994–2003 is shown in Fig. 5 forApril (left column) and August (right column). The ordinateshows the ratio of actual data to daily climatological data.All calculations were performed for a constant solar zenithangle of 63◦ for the whole period from 1994 to 2003. Am-plitude and standard deviation, as described above, are in-cluded. The maximum short-term variability of UV-radiationat 305 nm due to changes in total column ozone is 186% in



Table 1. The amplitude of the influencing factors total ozone, clouds and albedo on UV irradiance at 305 nm calculated as (maximum-minimum)/mean on the basis of daily data. Every single measurement during the time period from 1994 to 2003 is included. Standarddeviations are denoted below in italic.

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

ozone 2.11 2.56 1.79 1.86 1.41 1.17 1.10 1.02 1.10 1.27 1.49 1.530.38 0.56 0.37 0.33 0.22 0.20 0.17 0.17 0.20 0.20 0.28 0.27

clouds 1.23 1.22 1.19 1.51 1.47 1.24 1.58 1.15 1.46 1.21 1.16 –0.30 0.31 0.32 0.32 0.31 0.32 0.33 0.29 0.35 0.30 0.33–

albedo 0.27 0.26 0.31 0.32 0.21 0.15 0.12 0.14 0.13 0.18 0.24 0.230.04 0.05 0.04 0.06 0.04 0.03 0.03 0.02 0.02 0.04 0.04 0.04

Fig. 5. Box-plot presentation of the influence of total column ozone (top row) and albedo (bottom row) on variability of UV irradiance at305 nm in April and August, amplitude (A) and standard deviation (σ ) are shown in the respective top left corner. The boxes are bounded bythe 25% and 75% quartile, whiskers denote the 90% percentile. Normalized intensity is the ratio of the daily mean value of UV irradiance tothe 10-year monthly mean value.

April and 102% in August. Maximum albedo-induced vari-ability is 32% in April and 14% in August. Figure 6 showsthe evaluation for May, additionally considering the effectsof changing cloud cover. On a short time scale, clouds canalter the surface UV irradiance at 305 nm by a maximum of147%.

Model calculations were performed for each month. Ta-ble 1 summarizes the results obtained at 305 nm for theamplitude (representing maximum variability) and for thestandard deviation (resembling mean variability over the

averaging period): Ozone can affect UV irradiance on ashort time scale by up to 200% (mean variability up to50%). Clouds can cause variability of surface UV irradi-ance by up to about 150% (typical mean variability of around30%). Higher levels of attenuation are found in literature:Because of Sonnblick being a high altitude mountain site(3106 m a.s.l.), the layer-thickness of the observed cloudsis smaller, therefore the attenuation of UV-irradiance is re-duced (Blumthaler et al., 1996). Additionally, the increaseof albedo with altitude compensates for the attenuation by



Fig. 6. Box-plot presentation of the influence of total columnozone, clouds and albedo on variability of UV irradiance at 305 nmfor May. The boxes are bounded by the 25% and 75% quartile,whiskers denote the 90% percentile. Ozone and clouds are the dom-inating influences whereas the contribution of albedo is small.

Fig. 7. Maximum variation in monthly mean values of 305 nm irra-diance caused by total column ozone, clouds and albedo.

clouds to some extent. Simic (2006) found, that an increasein Albedo from 0.18 to 0.4 at Hoher Sonnblick may decreaseattenuation of UV irradiance on average by 21% at eight octacloud cover.

Albedo has the greatest influence on the variation of UVirradiance in April as this is the period of snow-melt, whichcauses the snowline (and thus the effective albedo) to changesignificantly on a relatively short time-scale. During summer,clouds contribute the major part of variability whilst ozonedominates in spring. This is explained by the strong variabil-ity of total column ozone during winter and spring and theenhanced cumulus convection during summer.

Figure 7 shows the maximum variability in monthly meanvalues of 305 nm irradiance induced by total column ozone,clouds and albedo. The effect of ozone on short-time vari-ability of monthly mean irradiance can be more than 50% inspring whereas the effect of clouds is largest during summerand early autumn. Variability caused by albedo ranges from3% to 13%.

Investigations by Arola et al. (2003) for the station So-dankyla (67◦ N, 191 m a.s.l.) in Finland show that the effectof total ozone on short-term variability of monthly mean UVirradiance at 305 nm can be almost 100%, rather than the50% found at Sonnblick. The greater effect of ozone foundat the arctic station Sodankyla may be explained through thestronger interannual variation of ozone concentration at highlatitudes. Variability caused by albedo in May in Sodankylais 21% (average at monthly level 7%) and 32% (average atmonthly level 6%) is found at the Sonnblick observatory.Thus variability caused by albedo is in the same range at bothstations.

Clouds can alter incoming UV-irradiance by up to 200%in Sodankyla compared to a maximum of 147% at Sonnblickduring the summer months: as a consequence of reduced



Table 2. Linear trends in relative departures of UV radiancefrom climatological monthly mean values, wavelengths from 305 to324 nm and CIE, solar zenith angles of 45◦, 55◦ and 65◦ are eval-uated for the time period 1994–2006. Trends are calculated withthe Theil-Sen method and are given in percent per decade, symbolsto the right indicate the significance of the trend [−: insignificant(α>0.1),+: significant (α≤0.1), *: highly significant (α≤0.01)].

SZA 305 nm 310 nm 315 nm 324 nm CIE

45◦−9.5+

−11.1+−10.5+

−9.8+−10.3+

55◦−23.9 * −19.9 * −15.6 * −14.4 * −19.5 *

65◦−3.8−

−5.7+−4.6+

−6.0+−4.4−

cloud layer thickness at high altitudes, the effect of attenu-ation is less pronounced at the latter station. At the Mediter-ranean site of Thessaloniki (40◦), Arola et al. found the vari-ability due to aerosols to be in the same range as variabilitydue to clouds, which is quite different at Sonnblick: the verysmall values of aerosol optical depth and the changes thereindo not cause more than 3% variation.

3.2 Long-term changes

Figure 8 shows relative departures of UV radiance fromclimatological monthly mean values. As stated in chapter2.3, only clear-sky spectra were included. Wavelengths of305 nm, 310 nm, 324 nm and erythemally weighted irradi-ances according to CIE at solar zenith angles of 45◦, 55◦ and65◦ are shown for the time period from 1994 to 2006. Dataare approximated with linear regression lines and the resultsare shown in percent per decade. Trends given in the upperright corner are calculated with the Theil-Sen method. Sym-bols in parenthesis indicate the significance of the trend [(−):insignificant (α>0.1), (+): significant (α≤0.1), (*): highlysignificant (α≤0.01)]. In Table 2, the results of the evalua-tions are summarized. Calculations are also performed withthe least-square method, and the obtained results coincidewell in cases with highly significant trends. In addition tothe MK-test, the series where checked for the minimum timeperiod necessary to reliably detect underlying trends of thefound magnitude at a significance level of at least 90%, asproposed by Weatherhead et al. (1998). It was shown, thatthe number of years required does not exceed the 13 years ofavailable data for all of the time series that exhibit a highlysignificant trend.

For a wavelength of 305 nm, downward trends of−9%/decat solar zenith angle 45◦ (90% confidence level) and−24%/dec at 55◦ (99% confidence level) are determined forthe period from 1994–2006. The trend is highly signifi-cant for 55◦ SZA, where the largest data set is available.Analysis of 310 nm, 315 nm and 324 nm basically deliversa comparable behaviour, however the trends are somewhatsmaller, i.e. the regression-line slopes becoming less negative

as the wavelength increases. Trends found in CIE integratedmonthly mean values consequently show smaller decreasesthan those found at 305 nm but they are steeper than those at315 nm and 324 nm.

Decrease of UV irradiance at short wavelengths wouldsuggest an increase in stratospheric ozone during the inves-tigation period from 1994 to 2006. Total ozone measure-ments performed with the Brewer spectrophotometer at theSonnblick observatory, however, do not show this trend, asthe small decrease of−3.67%/dec in monthly means thatinclude all ozone measurements, was found not to be sig-nificant using the Mann-Kendall trend-test. Following themethod of Weatherhead et al. (1998), the series would have toextend over more than 87 years to exhibit a trend at 90% sig-nificance level. Thus, further analysis of all other influencingfactors was required to explain those significant downwardtrends.

Checking for long term changes in time series of cloudcover and reconstructed albedo (calculated with Eq. 3) topossibly account for the changes in UV-irradiance also didnot identify significant linear trends over the period 1994–2006: The time-series of reconstructed albedo would haveto extend over more than 50 years for the downward trendof −3.9% per decade to be significant at a level of at least90%. Likewise, the small downward trend of−0.02%deccontained in total cloud cover observations during the investi-gated period is insignificant. There was no significant changedetected in partitioning of octa cloud cover either. On thecontrary, sunshine duration has increased by up to 9.6%/decof the 13-year monthly mean values during 1994–2006 atHoher Sonnblick. The largest increases were found duringspring and summer. In spring, the highest total column ozonevalues of the year are to be found: The 13-year monthlymean values of total column ozone during the first half-yearwere found to be about 14% higher than the respective valuesduring July to December. This explains the observed highlysignificant downward trends, since more UV-irradiance mea-surements during higher total column ozone concentrationswere included in the latter parts of the investigation period.At a solar zenith angle of 55◦, a significant increase of al-most 19%/dec in the number of available clear-sky spectrais present over the whole period. At the same time, the in-crease in the number of available clear-sky spectra duringJanuary to June only is almost 45%/dec. This may also ex-plain the fact that the 13-year monthly mean irradiances at305 and 310 nm and the CIE integrated irradiances are up to16% smaller during January to June than during the secondhalf-year. The same pattern can be recognized at 45◦ and65◦ SZA. However, at these SZAs fewer data are availableand the trends in sunshine duration are less pronounced.

Additionally, time series containing measurements underall-sky conditions were analysed for underlying trends. Ingeneral, they show the same behaviour as those containingclear-sky measurements only, revealing downward trends atseveral combinations of SZA and wavelength: At 55◦ SZA,



Fig. 8. Relative departures of UV irradiance from climatological monthly mean values, wavelengths of 305 nm (first row), 310 nm (secondrow), 324 nm (third row) and erythemally weighted irradiance according to CIE (fourth row) are shown for solar zenith angles of 45◦ (firstcolumn), 55◦ (second column) and 65◦ (third column), data from 1994 to 2006 are approximated with linear regression lines using the Theil-Sen method, symbols in parenthesis indicate trend significance [(−): insignificant (α>0.1), (+): significant (α≤0.1), (*): highly significant(α≤0.01) ].



downward trends of down to−23%/dec were found at 90%significance level for erythemally weighted irradiance as wellas for 310 nm under all-sky conditions. Downward trendsfound at 45◦SZA compared well to clear-sky conditions,the significance level being 90% for wavelengths of 290 nmand higher. However, caused by the enhanced variation dueto cloud-cover in all-sky data, fewer trends were found ashighly significant compared to the analysis of clear-sky spec-tra only. This is especially true with all-sky data at 65◦ SZA,where no significant trend was detected at all. Much longertime series (longer than 20 years on average) would havebeen needed to detect significant long-term changes underall-sky conditions.

In Sodankyla, Finland, spectral irradiance at 305 nm inApril decreases when the time period from 1993 to 2001 isinvestigated whereas it increases considering the period from1990 to 1997 (Lakkala et al., 2003). Investigations of long-term variability of monthly mean solar spectral irradiance at305 nm and 324 nm in Thessaloniki, Greece show positivetrends at both wavelengths for the period 1990–2006 (Baiset al., 2007). The investigated time period has a strong in-fluence on the calculated long-term change of spectral UVirradiance at Sonnblick observatory. When spectra mea-sured during the time period from 1994 to 2001 are analysed,mainly positive trends are found for different wavelengthsand solar zenith angles (Simic, 2005). Glandorf et al. (2005)evaluated several wavelengths and solar zenith angles of thelongest spectral UV irradiance measurements in Europe, inThessaloniki and Sodankyla. They conclude that even thelongest series are too short to show distinct trends due to thelarge variability of UV irradiance.

4 Summary

The influence of ozone, clouds, and surface albedo on spec-tral UV radiation at the Sonnblick observatory (3106 m) is in-vestigated using continuous spectral UV irradiance measure-ments and model calculations. Measurements at Sonnblickobservatory starting 1994 represent the longest time series ofspectral UV irradiance data in Austria and are used in thepresent investigation.

1. UV irradiance at wavelength 305 nm under clear skycondition is modelled and compared with the ac-tual measurements. In 95% of the investigated casesthroughout the period from 1994 to 2003, the calcu-lated UV irradiations are maximum 12% higher and 8%lower than actually measured irradiations. The determi-nation of the effective albedo at the observatory is essen-tial for a good correlation of measurement and model.The determination of the effective albedo uses the snowline altitude, the depth of the fresh fallen snow and thetime since last snowfall.

2. Measurements at Sonnblick observatory at 305 nmshow a pronounced influence of snowline height on sur-face UV irradiance. A decrease in snowline height from3000 m to 800 m a.s.l. enhances the UV irradiance by afactor of 1.24 for clear sky conditions and by a factor of1.7 for 8/8 cloud cover.

3. Short-term variability of UV irradiance at 305 nm be-cause of changes in ozone can be more than 200% (morethan 50% on average). Clouds can cause variability of150% or more (average 35%). Maximum variabilitycaused by albedo is 32% in April (average 6%) and 12–15% in the summer months (average 3%). In summerand autumn, total ozone and clouds strongly influencethe variability of UV radiation, whereas in winter andspring ozone has the more pronounced effect.

4. Spectral UV irradiance at 305 nm, 310 nm, 324 nm andCIE at solar zenith angles of 45◦, 55◦ and 65◦ from1994–2006, taken from measurements under clear-sky conditions, were analyzed for possible trends.Significant downward trends (99% confidence level)were found for solar zenith angle 55◦ at all wavelengths.At solar zenith angles of 45◦ and 65◦, downward trendsof UV irradiance were found for several wavelengths(90% confidence level). These trends can be explainedthrough an increase in the number of measurements un-der the influence of high total column ozone over theinvestigated period.

Acknowledgements.This work has been funded by the AustrianMinistry for Environment and by the Commission of the EuropeanCommunities, project ”Stratosphere-Climate Links with Emphasison the UTLS, Scout-03”.

Edited by:: J. Groebner

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